The narrative which follows can take whatever specific shape you like. We’re offering it in as generic terms as we care to, but utilizing whatever common themes make the most sense for a narrative specifically designed for a force of Custodes.
In the case of this campaign, the Custodes have received word from one of the eyes of the emperor that a long lost relic of the imperium that was has resurfaced on a shrine world. Unfortunately, the world has fallen into enemy hands. The Custodes must first eliminate an obstruction which would undermine the rest of their mission, then capture the relic then finally follow through with the final hammer blow. Try to think about what the valuable resource is before the campaign begins.
You are welcome to devise whatever specific elements you desire in order to tell this story on your tabletop. Much of this will be determined by the nature of the Antagonist, of course. For example, in Mission 1, the objective might be very unstable if the Antagonists are Orks, or it might have very powerful Warp resonance if the Antagonists are Chaos Daemons. Feel free to sprinkle in whatever rules you see fit to enhance the immersion of your games.
Check out to the associated Podcast episode, as well as additional content, here.
Check out the Masters of the Forge Rules Supplements & Materials page for additional, homebrew narrative rules you can use in your campaign or event!